Ultrasonic energy can be defined for our purposes as the form of wave energy transmitted through liquids at sonic velocity and ultrasonic (i.e. above about 15 kHz) frequency. This type of energy has been used previously in conjunction with chemical solutions and/or ultra-violet light to disinfect liquids containing harmful or undesirable micro-organisms including bacteria. It is also known to use ultrasonic energy in conjunction with high liquid pressures (typically at 1000 lb./square inch pressure) for the same purpose. Disinfection is generally understood to mean destroying or killing the micro-organisms and it is believed that the destruction is due to the violent local changes in liquid velocity and pressure associated with the ultrasonic waves.
There are situations, however, such as are involved in treating sewage or drinking water, where it is not convenient or desirable to employ either chemical solutions, ultra-violet light or high liquid pressures in conjunction with ultrasonic energy. In such applications, the liquid has to be treated continuously while flowing and the problem arises of how this is to be done.
If the liquid is treated while flowing in a pipe or channel, then the ultrasonic energy may be dissipated (i.e. lost by absorption in the containing walls of the pipe or channel). It is known to employ an ultrasonic energy source in the form disclosed by U.S. Pat. No. 3,966,120 wherein the liquid to which the energy is supplied is forced through a nozzle(s) onto the active surface of an ultrasonic transducer in an open atmosphere; the liquid then flows from the active surface as a sheet which eventually breaks up into drops and is collected in a suitable receiving tank. The advantage of this particular method is that the ultrasonic energy is transmitted to the liquid with only negligible loss since the liquid jet is surrounded by air rather than the solid walls of a pipe or channel.